The present invention relates to a power factor control system for AC induction motors and more particularly to such a system that electronically varies the duration of each cycle of AC power that is applied to an induction motor winding inversely with the phase lag angle of the winding current with respect to the applied voltage.
Such a system is described by F. Nola in the patent U.S. Pat. No. 4,052,648 issued Oct. 4, 1977. A high power resistor is placed in series with the motor winding to develop a voltage that is exactly in phase with the motor winding current. This resistor voltage and the line voltage are compared in an electronic circuit, e.g. including 8 operational amplifiers, that controls the firing of a bidirectional triode thyristor or TRIAC (Trademark of General Electric Company, Syracuse, N.Y.) that in turn controls delivery of the AC power to the motor winding. The above noted lag angle of the motor current is sensed and maintained essentially constant by this controller.
In a NASA publication entitled, Improved Power-Factor Controller, Brief No. MFS-23280, summer 1980, there is described a lower cost design than that in the patent. Three transformers and the high power resistor of the patent are eliminated and fewer components are used. However, two DC power supplies at +15 V and -15 V, respectively, operate from the AC power line and require two large filter capacitors. Also, only six operational amplifiers are employed.
These controller circuits have been adapted for insertion between a motor driven appliance (e.g. refrigerator, freezer, fan, etc.) and the home power outlet. They typically sell for $35. For a typical refrigerator operating at 10% duty factor, the use period required for the dollar savings in electrical energy to equal the cost of the power factor controller is at $0.08/Kwh almost 3 years. The installation of such a power factor controller with each of the several billion electric motors presently in use and with each of the approximately 50 million motors manufactured each year is clearly desirable and consistent with public policy for conserving energy, but is generally not yet cost effective in the home.
It is an object of the present invention to provide a power factor controller that is substantially simpler and less costly.
It is a further object of the present invention to provide such a controller that itself dissipates less energy and may be realized almost entirely in integrated circuit form.
It is yet a further object of the present invention to provide such a controller that is capable of being formed as an integral part of a power cord for an electrical appliance.
It is also an object of the present invention to provide such a controller that is suitable for permanent incorporation in a motor housing.